1. Field of the Invention
The present invention relates to a pattern evaluation apparatus and a method of pattern evaluation, and particularly to such an apparatus and a method which are applied, respectively, to inspect the defects of those patterns formed each on a photo mask or the like for use in the manufacture of semiconductor devices.
2. Description of the Related Art
The defects of photo masks used in the production of semiconductor devices with the photo lithographic technology applied are counted as one of major causes of decreasing the yield in the manufacture of large scale integrated circuits (LSI). Therefore, the development of devices to inspect such defects has thus far been actively carried out, with some then brought into practical services.
Conventional photo mask defect inspecting apparatus are roughly divided into two categories--one which follows a method wherein two chips having the same pattern formed thereon each are first inspected by separate detecting means, and the differences which the respective patterns exhibit in reference to each other are subsequently compared with one another, using proper defect detecting means; and another which is based on a method wherein each chip with a pattern provided thereon is observed, using detecting means, followed by comparing actual pattern data with pattern design data through appropriate defect detecting means to find out a defect. Since the former apparatus observes two chips which have the same pattern formed thereon each, should there exist an identical defect in each of these patterns, the device will fail to detect a defect portion where the defect is present, disclosing its shortcoming. However, said apparatus has a merit that a design data processing circuit is unnecessary, whereby the apparatus constitution may be simplified. Meanwhile, the latter apparatus is of a constitution wherein the inspection is effected with design data applied as a data base, thus showing on one hand a merit that defect detection is perfect and on the other hand a demerit that there takes place structural complexity. These defect inspecting apparatus undertake the detection of extra minute defects, thus having so far been experiencing the improvements not only for better resolutions of the optical systems but also of the comparison algorithm, the method of measured signal processing, and so forth.
On the other hand, of late, to avoid the trouble that microscopic dust particles will deposit on the surface of each sample, a glass substrate 101 which has once had a perfect pattern 102 provided thereon is brought into use with a pericle frame 103 stuck (bonded) thereto as shown in FIG. 1 with a several .mu.m thick thin film 104 called a pericle affixed thereto. Thus, it is required that the photo mask defect inspecting apparatus undertake pre-pericle sticking inspection and post-pericle sticking inspection for final check.
Generally with the optical system-mounted devices such as photo mask defect inspecting apparatus, it is known that increasing the numerical aperture (NA) of an objective lens is serviceable to improve the characteristic of resolving an image of each pattern to be observed. Recent photo mask defect inspecting devices are required capable of detecting a defect of less than 0.3 .mu.m, wherein the optical systems which have so far been brought into service with NA=approximately 0.5 to 0.6 selected, are needed of late to go into use with NA=approximately 0.7 to 0.8. These devices raise no problem over the inspection of pericle free photo masks. However, as the numerical aperture (NA) of an objective lens grows as shown in FIG. 2, the photo mask defect inspecting apparatus have their serviceability limited to the inspection areas narrower than ever. Namely, they are limited to inspect only the area far remote from the pericle frame. This is because, as illustrated in FIG. 2, the pericle frame interrupts the rays of light, with the light beam getting asymmetric, whereby there occurs a change of the luminous energy. Since the size of a pericle frame, the dimensions of a pattern forming area, etc. depend, to a certain degree, on device design particulars and what is the peripheral apparatus involved (a stepper, etc. for example), it is desired that the defect inspecting apparatus be capable of inspecting pericle fitted photo masks over their wide possible areas with the high accuracy of detection kept unchanged.
As a means to solve such problems, a method is available wherein the problems are settled with the NA of an optical system on the side of observation lessened. But the effect in the fall of detecting accuracy stemming from provision of a smaller NA is not nullified.